hydronic radiant floor heating

I've got a floor heating system heated by nat gas water heater with plastic tubing. It has a bladder pressure buffer tank incorporated and autobleeder. On the city water inlet side there is a pressure regulator and what appears to be a check valve/relief valve "upstream" from that. The relief valve has begun to leak (about 1 drop per minute) It did this about 4 years ago and I replaced it. Now that it is leaking again, my question is, should I be checking air pressure on the buffer tank or bleeding the system or what. I can't seem to find any maintenance procedures for this system. The "vent" outlet on the relief valve is not threaded for npt. It looks like it is for "sweat in" copper pipe to drain, but it's too big for 1/2" copper and too small for 3/4. There is no spec tag on this valve other than the fact that it has 1/2" mpt fittings on each end. Anybody got any ideas? Thank You. Dennis